As far as I am aware, there is no conventional method for centering the button. However, I have discovered an easy cure. It is not the proper way, but it works.
You can experiment with the following:
col1, col2, col3 , col4, col5 = st.beta_columns(5)
with col1:
pass
with col2:
pass
with col4:
pass
with col5:
pass
with col3 :
center_button = st.button('Button')